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Author: Subject: Animal, Vegetable or Mineral?
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[*] posted on 5-18-2008 at 10:07 AM
Animal, Vegetable or Mineral?

Wandering the other day near San Juanico, we came across two curious features in arroyo sediment walls that I have not been able to identify. They are presently mineral but perhaps started out as something else. Please look at the photos and let me know if they are familiar to you.
The first ones are orange discs verticle in the wall. They look somewhat flat clamlike but not their orientation nor other normal bivalve features.
The second looks like a giant fallen redwood tree but nothing I find on the web for fossilized trees looks remotely like this. And the cross section pieces don’t look like tree to me.
Thank you very much if you can shed some light on these items I find interesting!
Here is the address to the photos. http://gallery.mac.com/jan_mikey/100087
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[*] posted on 5-18-2008 at 12:02 PM


I am a paleontologist, so hopefully this will help. The photos aren’t the best, but here goes:

The objects in the wall are probably burrows (known as trace fossils) they may have been dug by clams or shrimp. After the burrow is abandoned, it fills up with different colored sediment, or sediment that oxidizes at a different rate that the surrounding sediment. They may also be root casts, left behind after a plant died (see cast explanation below)

The objects eroded out of the sediment could be clam casts or simply what are known as armored mud balls. I’d have to see a more close up view if several of them. Clam casts form when a clam dies and the shell dissolves and leave a void (mold) in the sediment, this fills up with other sediment leaving a cast of the clam….think of the human casts from Mt Vesuvius. Armored mud balls are simply chunks of sediment that are redeposited in a new location, as they travel on their way they become rounded, and sometimes pick up additional muddy sediment around then like a snow ball rolling down a hill.

If they are clam casts, you will see a pattern in the shapes; i.e. many will look the same, you also may see textures on the outside from the ridges the clamshell had. If they are mud balls the shapes will all be different.

The formation that looks like a giant fallen tree is just closely spaced layers of different colored sediment, and probably different textured sediment. The dark yellow is probably sandier, and the blue-gray is probably more silty to clayey. Different textures (sand-silt-clay) will erode at different rates, leading to its corrugated look when exposed to the elements. Wood does the same thing, the summer growth (thik and lighter) is softer and the winter growth (thinner and darker) and is harder.

If you looks closely at some of the layered sediment, mainly in the siltier and clayier layers, you may see some fish scales and fish bones. Crack the layers open like the pages of a book with a hammer. The scales will be dark brown spots up to ~ 1/2 inch in diameter, and look just like fish scale today. The bones will also be dark brown .



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[*] posted on 5-18-2008 at 12:12 PM


TdB, Thank you very much for the explanations. I will go back and look again with this additional information.
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